Organic Light Emitting Diodes (OLEDs) have come to the fore as the state-of-the-art technology for visual displays and lighting. OLEDs are desirable as they are light weight, flexible, provide better contrast and possess large viewing angle. OLEDs are also more power efficient than traditional lighting sources and thus their wide adoption can alleviate significantly the strain on current energy demand because lighting alone constitutes about 20% of energy consumption worldwide.
The “first generation” OLEDs were based on organic fluorescent emitters whose efficiency was intrinsically capped at 25% due to only being able to recruit singlet excitons. The “second generation” OLEDs employed organometallic phosphorescent emitters, which harvest both singlet and triplet excitons for emission due to the enhanced intersystem crossing (ISC) mediated by the large spin-orbit coupling of heavy metals such as iridium(III) and platinum(II). Despite their highly desirable performance characteristics, the rarity of these metals, their high cost and their toxicity are important detracting features that inhibit large-scale, worldwide adoption of OLED technology.
The “third generation” OLEDs were recently first reported by Adachi and co-workers. His group demonstrated how small organic molecules, emitting via a thermally activated delayed fluorescence (TADF) mechanism, could be integrated into OLEDs and exhibit very high efficiencies as, like with phosphorescent emitters, both singlet and triplet excitons are recruited for emission (Reference 1). Thus, TADF-based OLEDs address the key detracting features endemic to “second generation” OLEDs while retaining their advantages (Reference 2).
The principle of TADF relies on a small energy gap between the lowest singlet and triplet excited states (ΔEST). Under these conditions, the electrons in the triplet state can return to the singlet state by reverse intersystem crossing (RISC) using thermal energy, followed by radiative fluorescence (Reference 1a). The small ΔEST is realized by spatial separation between HOMO and LUMO to minimize the electronic repulsion between these orbitals. A large number of organic TADF emitters have been reported to date. They can make use of donor and acceptor moieties of various types within the molecule to achieve the desired small energy gap between the lowest singlet and triplet excited states (ΔEST). The majority of these molecules are based on a twisted intramolecular charge transfer (TICT) design in which the donor and acceptor moieties are designed to be nearly orthogonal to each other (References 1a, 1c and 3).
However, current OLEDs, including TADF-OLEDs, still employ air sensitive electrodes requiring encapsulation, are vacuum deposited limiting the size of the device, and possess a complex multi-layer architecture that add to the cost of fabrication.
Single-layer solid-state light-emitting electrochemical cells (LEECs) have received much recent attention for their potential to address these negative design features found in OLEDs (Reference 4). The emitters in LEECs are frequently ionic transition metal complexes, the most popular and highest performing class of which are cationic iridium(III) complexes.
As with their use in OLEDs the use of rare heavy metal complexes in LEECs presents challenges. As an alternative to small molecule emitters for LEECs can include conjugated polymers together with ion transport material and inorganic salts such as LiOTf. Recently a first example of an operational LEEC with a small-molecule organic cyanine dye based fluorophore as an alternative to iridium based complexes has been disclosed (Reference 5).
Despite the progress made there is a need to provide improved and alternative compounds for use in display and lighting uses, such as in light emitting electrochemical cells (LEECs).